The kinetics of the reactions of thrombin and collagen with platelets will be investigated, following secretion of CA2 ion or ATP as a measure of the reactions. The enzymic nature of the thrombin reaction will be evaluated by use of competitive inhibitors and active site modified thrombin. The role of thrombin binding will be evaluated using I125-thrombin to measure number of binding sites, binding constants, rates of binding and rates of dissociation.